1. Field of the Art
The present invention relates to a method for treating refined mechanical pulp and thermo mechanical pulp with ozone, wherein the pulp is treated in an ozone reactor immediately after one or more processing steps in a disc refiner and thereafter is treated with high consistency in a maturation reactor.
More particularly the present invention relates to a method of treating refined mechanical pulp (Refined Mechanical Pulp, RMP) and thermo mechanical pulp (Thermo Mechanical Pulp, TMP) with ozone between two associated refining steps.
2. Statement of Prior Art
It is previously known that in treating mechanical pulps with ozone the properties of the pulp can be improved considerably, see in this respect for example Norsk Skogindustri No. 2 (1968) 46, No. 3 (1972) 61, No. 5 (1971) 135, No. 10 (1973) 274, No. 6 (1974) 165 and NO-PS 115 279.
Further, in Norwegain Patent Application No. 75 3661 there is described a method and an apparatus for treating finely divided pulp with ozone gas without overpressure, in which the ozone treated pulp is subjected to a low consistency maturation. A further development of this method and apparatus is described in Norwegian Patent Application No. 77 1473, according to which the ozone treated pulp is brought direct into a high consistency maturation reactor, which is so designed that the total processing time represented by the gas phase reaction time the maturation time is considerably reduced. Accordingly, also the size of the process equipment is reduced, a combined maturation and bleaching of the pulp being accomplished without auxiliary equipment.
In NO-PS 131 996 there is described a method for treating paper pulp with ozone which is to the effect that the pulp is refined in a disc refiner at high consistency, the pulp immediately thereafter being subjected to an ozone treatment. Thereby is achieved a pulp having an especially appropriate consistency and a favourable physical condition --i.e. a light and fluffy or so-called fluffed pulp --for the ozone treatment without the use of particular dewatering and fluffer equipment.
However, this Patent Specification renders no concrete instructions for what freeness or what dewatering properties the pulp should have during the ozone treatment or how many processing stages the pulp should go through in the disc refiners. Nor does this Patent Specification provide any instructions for a treatment of the pulp subsequent to the ozone treatment, which in view of the preceding treatment consumes considerably less energy and gives a finished pulp having further favourable tear and tensile strength qualities.
In Norwegian Patent Application No. 77 1471 there is disclosed a method for treating pulp with ozone, which suggests that the finished defibrated pulp be fractionated prior to the ozone treatment thereby taking advantage of the condition that a pulp obtains a larger tear strength, the higher the freeness of the pulp is during the ozone treatment.
Further, Norwegian Patent Application No. 77 1472 gives instructions for a method which takes advantage of the above-mentioned condition in connection with reject pulps.
If the prior art was taken as a starting point for achieving a pulp having such a freeness number that it is suited for ozone treatment, the following two procedures would be followed.
(1) The pulp is fractionated, a fact which entails a dilution of the pulp to so-called screening consistency, which usually means solids content of approximately 1%, to fractionate the coarse fraction. This is then subjected to dewatering and fulffing before the ozone treatment.
(2) A reject pulp is used, which per definition is a coarse pulp, and this pulp is concentrated and fluffed before the ozone treatment, whereafter the ozonized pulp is passed through a conventional reject treatment.
As described in Norwegian Patent Applications Nos. 77 1471 and 77 1472, an ozone treatment of a coarse pulp achieves an energy reduction compared to the same treatment of a more finely divided pulp, the necessary energy for the dewatering/pressing being reduced with increasing freeness of the pulp. The total energy necessary for dewatering a pulp having a freeness of approximately 100 csf (Canadian Standard Freeness) to approximately 35% TS (solids) lies in the range of 60 kwh/odt (oven dry ton), and only a minor part of this energy can be saved in the use of a coarser pulp produced by fractionation or represented by reject pulp.
Thus, the object of the present invention is to provide a method which makes a complete integration of the ozone treatment in the pulp production process possible, and which affords a substantial reduction of the energy consumption when the entire pulp production process is looked upon as a whole. Further, an object of the invention is to give instructions for a method which permits a substantial simplification of the equipment involved in the process-technical plant.